This invention relates to data communications networks and more specifically to a device for measuring the power level of signals in such a network.
A typical data communications network couples a data processing system, or some other source of data signals in digital form, at a central site with one or more terminals, data processing systems or other input/output devices that use digital data at various remote sites. In such a network, the data processing system at the central site includes means for transmitting a message in digital form that contains an address of and data for a particular remote device or system. A "modem" (i.e., a modulator-demodulator) encodes the digital data and modulates a carrier signal with the encoded digital data and transmits the modulated carrier signal over a telephone line to the remote sites, whereupon other modems at the remote sites demodulate and decode the incoming data thereby to receive the digital data for use by the selected device or system. When the device or system at the remote site generates data in digital form, the associated modem encodes that data and modulates a carrier signal with the encoded data for transmission back over the telephone line to the central site where the central site modem demodulates and decodes the signal to recover the digital data.
In some networks, it is possible to establish parallel communication paths through conventional telephone circuits. One such arrangement is shown in the foregoing U.S. Pat. Application Ser. No. 665,256. An analog bridge is interposed between the modem at the central site and the primary telephone connections to the remote sites. Direct access arrangements including dial telephones are then connected to each other output port on the bridge to establish communication paths to auxiliary inputs of particular remote sites. If there is a problem on the primary telephone connection, a person at the central site can establish communications through the secondary telephone lines and a corresponding output port.
In other applications it may be desirable to use such bridges, singly or in cascade, to increase the number of telephone lines that are connected to a modem. For example, a typical bridge may contain a connection for a private line plus eight additional lines. It is possible therefore to connect eight additional private lines to the modem through this device, so long as transmissions from the central site may be in parallel to all nine private lines and the incoming signals, in parallel from all the lines, can be summed in analog fashion, because such bridges performs no multiplexing functions.
Oftentimes network problems can be diagnosed by measuring the power of the signals on the telephone line. In prior data communications networks, complex measuring devices have been used to make these measurements. However, these various measuring devices are often difficult to read in a dynamic environment such as is encountered in a multi-point network that operates over telephone lines. Moreover, they are sometimes difficult to use, especially if it is desirable to make any adjustments of the bridge to compensate for average signal levels.
Therefore, it is an object of this invention to provide a device for measuring the power of a modulated carrier signal in a data communications network.
Another object of this invention is to provide a device for measuring the power of a signal on a telephone line that is easy to use and which provides easily readable results.
Still another object of this invention is to provide a line level measuring device that is adapted for use in conjunction with an analog bridge in a data communications network.
Still another object of this invention is to provide a line level measuring device that includes circuits for testing the operation of the device.